I am trying to install itunes and firefox however when I Download the dmg files it says the archetecture does not support the versions of the files I have downloaded. How can I fix this?

I have a mac mini with osx v 10.5.8 and I am trying to install itunes and firefox however when I Download the dmg files it says the archetecture does not support the versions of the files I have downloaded. How can I fix this so that I can install versions of programs that will allow me to utilize my mac? I have all the software updates downloaded and installed bu the issue persists.